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Parameter
Dimension
Value
Fuel type
Wood logs
Length of the wood logs
mm
See Table 2.2.
Moisture content
%
≤
20%
MJ/kg
>13.9
Low calorific value of the fuel (LHV)
kWh/kg
>3.9
Table 2.3. Proximate parameters of the recommended fuel – predried
and chopped wood logs;
EXPLANATION: one of the most important parameters of the fuel is its
calorific value (its heat content). The calorific value (also known as heating
value) is function of the moisture content of the wood logs (the moisture is
mechanically conserved water in the fuel. The biomass is a natural product and
the moisture content of a raw material could be 40 to 60% (the just cut down
material can contain up to 80%).
In order to achieve economic operation of the boiler and to achieve
efficient combustion process, it is necessary to supply fuel with minimal water
content, for pyrolisis types of boilers it is recommended the moisture content to
be up to 20%, which is practically realized by natural drying of the material (at
ambient conditions) for a relatively long period of time (at least for a year),
during the summer period the drying is high intensified. The fuel should be
protected from accident moistening (from example by rain or snow). In order to
achieve uniform and intensive during it is highly recommended to chop the wood
logs into smaller pieces. However, it the wood logs are stored for a longer
period (more than two years), its calorific value is also decreased due to
devolatilization of the biomass;
The so called Low Calorific Value of the fuel (LHV) – the heat energy,
achieved at combustion, but the latent heat of the water vapor in the flue gases
is not retrieved could be calculated from the following equation:
(
)
100
44
.
2
100
*
r
r
W
W
HHV
LHV
−
−
=
, [MJ/kg], where :
LHV – Low Heating Value of the fuel, [MJ/kg];
HHV = 18.0 [MJ/kg] – High Heating Value of the fuel (this is the energy of the
fuel, which includes the latent heat of evaporation of the water vapor in the flue
gases, i.e. the water has been condensed);
The dependency of the Low Heating Value as function of the moisture
content W
r
is given on the following figure:
